Oximetry is the measurement of the oxygen level status of blood. Early detection of low blood oxygen level is critical in the medical field, for example in critical care and surgical applications, because an insufficient supply of oxygen can result in brain damage and death in a matter -of minutes. Pulse oximetry is a widely accepted noninvasive procedure for measuring the oxygen saturation level of arterial blood, an indicator of oxygen supply. A pulse oximetry system consists of a sensor applied to a patient, a pulse oximeter, and a patient cable connecting the sensor and the pulse oximeter.
The pulse oximeter may be a standalone device or may be incorporated as a module or built-in portion of a multiparameter patient monitoring system, which also provides measurements such as blood pressure, respiratory rate and EKG. A pulse oximeter typically provides a numerical readout of the patient""s oxygen saturation, a numerical readout of pulse rate, and an audible indicator or xe2x80x9cbeepxe2x80x9d that occurs in response to each pulse. In addition, the pulse oximeter may display the patient""s plethysmograph, which provides a visual display of the patient""s pulse contour and pulse rate.
FIG. 1 illustrates a prior art pulse oximeter 100 and associated sensor 110. Conventionally, a pulse oximetry sensor 110 has LED emitters 112, typically one at a red wavelength and one at an infrared wavelength, and a photodiode detector 114. The sensor 110 is typically attached to an adult patient""s finger or an infant patient""s foot. For a finger, the sensor 110 is configured so that the emitters 112 project light through the fingernail and through the blood vessels and capillaries underneath. The LED emitters 112 are activated by drive signals 122 from the pulse oximeter 100. The detector 114 is positioned at the fingertip opposite the fingernail so as to detect the LED emitted light as it emerges from the finger tissues. The photodiode generated signal 124 is relayed by a cable to the pulse oximeter 100.
The pulse oximeter 100 determines oxygen saturation (SpO2) by computing the differential absorption by arterial blood of the two wavelengths emitted by the sensor 110. The pulse oximeter 100 contains a sensor interface 120, an SpO2 processor 130, an instrument manager 140, a display 150, an audible indicator (tone generator) 160 and a keypad 170. The sensor interface 120 provides LED drive current 122 which alternately activates the sensor red and IR LED emitters 112. The sensor interface 120 also has input circuitry for amplification and filtering of the signal 124 generated by the photodiode detector 114, which corresponds to the red and infrared light energy attenuated from transmission through the patient tissue site. The SpO2 processor 130 calculates a ratio of detected red and infrared intensities, and an arterial oxygen saturation value is empirically determined based on that ratio. The instrument manager 140 provides hardware and software interfaces for managing the display 150, audible indicator 160 and keypad 170. The display 150 shows the computed oxygen status, as described above. The audible indicator 160 provides the pulse beep as well as alarms indicating desaturation events. The keypad 170 provides a user interface for such things as alarm thresholds, alarm enablement, and display options.
Computation of SpO2 relies on the differential light absorption of oxygenated hemoglobin, HbO2, and deoxygenated hemoglobin, Hb, to determine their respective concentrations in the arterial blood. Specifically, pulse oximetry measurements are made at red and IR wavelengths chosen such that deoxygenated hemoglobin absorbs more red light than oxygenated hemoglobin, and, conversely, oxygenated hemoglobin absorbs more infrared light than deoxygenated hemoglobin, for example 660 nm (red) and 905 nm (IR).
To distinguish between tissue absorption at the two wavelengths, the red and IR emitters 112 are provided drive current 122 so that only one is emitting light at a given time. For example, the emitters 112 may be cycled on and off alternately, in sequence, with each only active for a quarter cycle and with a quarter cycle separating the active times. This allows for separation of red and infrared signals and removal of ambient light levels by downstream signal processing. Because only a single detector 114 is used, it responds to both the red and infrared emitted light and generates a time-division-multiplexed (xe2x80x9cmodulatedxe2x80x9d) output signal 124. This modulated signal 124 is coupled to the input of the sensor interface 120.
In addition to the differential absorption of hemoglobin derivatives, pulse oximetry relies on the pulsatile nature of arterial blood to differentiate hemoglobin absorption from absorption of other constituents in the surrounding tissues. Light absorption between systole and diastole varies due to the blood volume change from the inflow and outflow of arterial blood at a peripheral tissue site. This tissue site might also comprise skin, muscle, bone, venous blood, fat, pigment, etc., each of which absorbs light. It is assumed that the background absorption due to these surrounding tissues is invariant and can be ignored. Thus, blood oxygen saturation measurements are based upon a ratio of the time-varying or AC portion of the detected red and infrared signals with respect to the time-invariant or DC portion:
RD/IR=(RedAc/RedDC)/(IRAC/IRDC)
The desired SpO2 measurement is then computed from this ratio. The relationship between RD/IR and SpO2 is most accurately determined by statistical regression of experimental measurements obtained from human volunteers and calibrated measurements of oxygen saturation. In a pulse oximeter device, this empirical relationship can be stored as a xe2x80x9ccalibration curvexe2x80x9d in a read-only memory (ROM) look-up table so that SpO2 can be directly read-out of the memory in response to input RD/IR measurements.
Pulse oximetry is the standard-of-care in various hospital and emergency treatment environments. Demand has lead to pulse oximeters and sensors produced by a variety of manufacturers. Unfortunately, there is no standard for either performance by, or compatibility between, pulse oximeters or sensors. As a result, sensors made by one manufacturer are unlikely to work with pulse oximeters made by another manufacturer. Further, while conventional pulse oximeters and sensors are incapable of taking measurements on patients with poor peripheral circulation and are partially or fully disabled by motion artifact, advanced pulse oximeters and sensors manufactured by the assignee of the present invention are functional under these conditions. This presents a dilemma to hospitals and other caregivers wishing to upgrade their patient oxygenation monitoring capabilities. They are faced with either replacing all of their conventional pulse oximeters, including multiparameter patient monitoring systems, or working with potentially incompatible sensors and inferior pulse oximeters manufactured by various vendors for the pulse oximetry equipment in use oat the installation.
Hospitals and other caregivers are also plagued by the difficulty of monitoring patients as they are transported from one setting to another. For example, a patient transported by ambulance to a hospital emergency room will likely be unmonitored during the transition from ambulance to the ER and require the removal and replacement of incompatible sensors in the ER. A similar problem is faced within a hospital as a patient is moved between surgery, ICU and recovery settings. Incompatibility and transport problems are exacerbated by the prevalence of expensive and non-portable multiparameter patient monitoring systems having pulse oximetry modules as one measurement parameter.
The Universal/Upgrading Pulse Oximeter (UPO) according to the present invention is focused on solving these performance, incompatibility and transport problems. The UPO provides a transportable pulse oximeter that can stay with and continuously monitor the patient as they are transported from setting to setting. Further, the UPO provides a synthesized output that drives the sensor input of other pulse oximeters. This allows the UPO to function as a universal interface that matches incompatible sensors with other pulse oximeter instruments. Further, the UPO acts as an upgrade to existing pulse oximeters that are adversely affected by low tissue perfusion and motion artifact. Likewise, the UPO can drive a SpO2 sensor input of multiparameter patient monitoring systems, allowing the UPO to integrate into the associated multiparameter displays, patient record keeping systems and alarm management functions.
One aspect of the present invention is a measurement apparatus comprising a sensor, a first pulse oximeter and a waveform generator. The sensor has at least one emitter and an associated detector configured to attach to a tissue site. The detector provides an intensity signal responsive to the oxygen content of arterial blood at the tissue site. The first pulse oximeter is in communication with the detector and computes an oxygen saturation measurement based on the intensity signal. The waveform generator is in communication with the first pulse oximeter and provides a waveform based on the oxygen saturation measurement. A second pulse oximeter is in communication with the waveform generator and displays an oxygen saturation value based on the waveform. The waveform is synthesized so that the oxygen saturation value is generally equivalent to the oxygen saturation measurement.
In another aspect of the present invention, a measurement apparatus comprises a first sensor port connectable to a sensor, an upgrade port, a signal processor and a waveform generator. The upgrade port is connectable to a second sensor port of a physiological monitoring apparatus. The signal processor is configured to compute a physiological measurement based on a signal input to the first sensor port. The waveform generator produces a waveform based on the physiological measurement, and the waveform is available at the upgrade port. The waveform is adjustable so that the physiological monitoring apparatus displays a value generally equivalent to the physiological measurement when the upgrade port is attached to the second sensor port.
Yet another aspect of the present invention is a measurement method comprising the steps of sensing an intensity signal responsive to the oxygen content of arterial blood at a tissue site and computing an oxygen saturation measurement based on the intensity signal. Other steps are generating a waveform based on the oxygen saturation measurement and providing the waveform to the sensor inputs of a pulse oximeter so that the pulse oximeter displays an oxygen saturation value generally equivalent to the oxygen saturation measurement.
An additional aspect of the present invention is a measurement method comprising the steps of sensing a physiological signal, computing a physiological measurement based upon the signal, and synthesizing a waveform as a function of the physiological measurement. A further step is outputting the waveform to a sensor input of a physiological monitoring apparatus. The synthesizing step is performed so that the measurement apparatus displays a value corresponding to the physiological measurement.
A further aspect of the present invention is a measurement apparatus comprising a first pulse oximeter for making an oxygen saturation measurement and a pulse rate measurement based upon an intensity signal derived from a tissue site. Also included is a waveform generation means for creating a signal based upon the oxygen saturation measurement and the pulse rate measurement. In addition, there is a communication means for transmitting the signal to a second pulse oximeter.
Another aspect of the present invention is a measurement apparatus comprising a portable portion having a sensor port, a processor, a display, and a docking connector. The sensor port is configured to receive an intensity signal responsive to the oxygen content of arterial blood at a tissue site. The processor is programmed to compute an oxygen saturation value based upon the intensity signal and to output the value to the display. A docking station has a portable connector and is configured to accommodate the portable so that the docking connector mates with the portable connector. This provides electrical connectivity between the docking station and the portable. The portable has an undocked position separate from the docking station in which the portable functions as a handheld pulse oximeter. The portable also has a docked position at least partially retained within the docking station in which the combination of the portable and the docking station has at least one additional function compared with the portable in the undocked position.
A further aspect of the present invention is a measurement apparatus configured to function in both a first spatial orientation and a second spatial orientation. The apparatus comprises a sensor port configured to receive a signal responsive to a physiological state. The apparatus also has a tilt sensor providing an output responsive to gravity. In addition, there is a processor in communication with the sensor port and the tilt sensor output. The processor is programmed to compute a physiological measurement value based upon the signal and to determine whether the measurement apparatus is in the first orientation or the second orientation based upon the tilt sensor output. A display has a first mode and a second mode and is driven by the processor. The display shows the measurement value in the first mode when the apparatus is in the first orientation and shows the measurement value in the second mode when the apparatus is in the second orientation.
Another aspect of the present invention is a measurement method comprising the steps of sensing a signal responsive to a physiological state and computing physiological measurement based on the signal. Additional steps are determining the spatial orientation of a tilt sensor and displaying the physiological measurement in a mode that is based upon the determining step.